Contrast Tuning in Auditory Cortex

Abstract

The acoustic features useful for converting auditory information into perceived objects are poorly understood. Although auditory cortex neurons have been described as being narrowly tuned and preferentially responsive to narrowband signals, naturally occurring sounds are generally wideband with unique spectral energy profiles. Through the use of parametric wideband acoustic stimuli, we found that such neurons in awake marmoset monkeys respond vigorously to wideband sounds having complex spectral shapes, preferring stimuli of either high or low spectral contrast. Low contrast-preferring neurons cannot be studied thoroughly with narrowband stimuli and have not been previously described. These findings indicate that spectral contrast reflects an important stimulus decomposition in auditory cortex and may contribute to the recognition of acoustic objects.

Non-isomorphism in efficient coding of complex sound properties

“…For example, while Barbour and Wang (2003) report neural sensitivity in primary auditory cortex to levels of spectral contrast (non-isomorphic), several other reports document neural encoding of the gross frequency characteristics of a stimulus (isomorphic; e.g., Wang et al, 1995). More confident speculation concerning underlying sensorineural processing responsible for the present findings must await additional behavioral and physiological experiments.…”

“…Strictly speaking, vibrato causes the full spectrum to shift up and down in absolute frequency while maintaining constant spectral shape. Thus, vibrato-induced changes are similar to encoding spectral shape across varying absolute frequencies in physiological studies of cortical encoding (Barbour and Wang, 2003). Vibrato was varied in 18 nearly logarithmic steps from 7.5-19 Hz, with step sizes normed in pilot studies to share equivalent JND spacing as achieved for AD and SS series.…”

“…Consistent with this principle, Wang (2007) describes "non-isomorphic" transformations that occur progressively along the ascending auditory pathway, making neural representations "further away from physical (acoustical) structures of sounds, but presumably closer to internal representations underlying perception" (p. 92). Examples of non-isomorphic representations in auditory cortex include encoding spectral shape across varying absolute frequencies (Barbour and Wang, 2003), gross representation of rapid change in click trains with short inter-click intervals versus phase-locking to trains with slower inter-click intervals (Lu and Wang, 2000;Lu et al, 2001), pitch versus individual frequency components Wang, 2005, 2006), and different components of auditory scenes (Nelken and Bar-Yosef, 2008).…”

Fine frequency tuning in monkey auditory cortex and thalamus

“…In the present study, we presented pure tones one at a time, in contrast to simultaneously presented multiple tones ("chord" stimuli) in the study by Bitterman et al (2008). It is quite likely that we would have observed even smaller bandwidths had we used stimuli of higher density such as random spectral stimuli (Barbour and Wang 2003) or random chord stimuli (Bitterman et al 2008;Blake and Merzenich 2002).…”

Probabilistic Encoding of Vocalizations in Macaque Ventral Lateral Prefrontal Cortex

“…Behavioral research suggests that primates use multiple features to discriminate among calls, including the interpulse interval in noisy calls, the overall amplitude envelope and the location of inflections in the frequency contour Le Prell and Moody, 2000;Ghazanfar et al, 2001aGhazanfar et al, , 2002. Previous research on the primate auditory cortex suggests that the frequency (Barbour and Wang, 2003) or temporal contrast of sounds is being represented. Another possibility, not mutually exclusive to the others, is that the auditory system is performing a dynamic analysis of the sound, taking into account the time-varying structure of its spectral features, which can be modeled by a hidden Markov model (HMM) (Rabiner, 1989).…”